1. Field of the Invention
The present invention relates generally to the field of emissions control and, in particular to a new and useful method and/or system by which to control various types of corrosion and/or precipitation issues in at least a portion of a wet flue gas desulfurization (WFGD) scrubber system. In one embodiment, the method and/or system of the present invention relies on the supply of at least one reducing agent to the slurry of a wet flue gas desulfurization scrubber to lower the oxidation reduction potential in the absorber slurry contained within the wet flue gas desulfurization scrubber. In still another embodiment, the method and/or system of the present invention control the oxidation-reduction potential in at least one bleed stream of an absorber slurry, filtrate, and/or solution from a wet flue gas desulfurization scrubber.
2. Description of the Related Art
A variety of SO2 control processes and technologies are in use and others are in various stages of development. Commercialized processes include wet, semidry (slurry spray with drying) and completely dry processes. The wet flue gas desulfurization (WFGD) scrubber is the dominant worldwide technology for the control of SO2 from utility power plants, with approximately 85 percent of the installed capacity, although the dry flue gas desulfurization (DFGD) systems are also used for selected lower sulfur applications.
Wet scrubbing processes are often categorized by reagent and other process parameters. The primary reagent used in wet scrubbers is limestone. However, any alkaline reagent can be used, especially where site-specific economics provide an advantage. Other common reagents are lime (CaO), magnesium enhanced lime (MgO and CaO), ammonia (NH3), and sodium carbonate (Na2CO3).
A number of the wet processes are also classified as either non-regenerable or regenerable systems. In non-regenerable systems, the reagent in the scrubber is consumed to directly generate a byproduct containing the sulfur, such as gypsum. In regenerable systems, the spent reagent is regenerated in a separate step to renew the reagent material for further use and to produce a separate byproduct, such as elemental sulfur. The dominant limestone and lime reagent systems used today are non-regenerable. In many cases the regenerable systems have been retrofitted with non-regenerable limestone or lime reagent systems to reduce costs and improve unit availability.
As known to those of skill in the art, the most common WFGD absorber module is the spray tower design (see, e.g., Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright 2005, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., particularly Chapter 35—Sulfur Dioxide Control, the text of which is hereby incorporated by reference as though fully set forth herein). In the most common WFGD set-up the flue gas enters the side of the spray tower at approximately its midpoint and exits through a transition at the top. The upper portion of the module (absorption zone) provides for the scrubbing of the flue gas to remove the SO2 while the lower portion of the module serves as an integral slurry reaction tank (also frequently referred to as the recirculation tank (or absorber recirculation tank) and oxidation zone) to complete the chemical reactions to produce gypsum. The self-supporting absorber towers typically range in diameter from 20 feet to 80 feet (6 meters to 24 meters) and can reach 150 feet (46 meters) in height. In some designs, the lower reaction tank is flared downward to provide a larger diameter tank for larger slurry inventory and longer retention time. Other key components include the slurry recirculation pumps, interspatial spray headers and nozzles for slurry injection, moisture separators to minimize moisture carryover, oxidizing air injection system, slurry reaction tank agitators to prevent settling, and the perforated tray to enhance SO2 removal performance. An exemplary illustration of a WFGD is shown in FIG. 1.
It has been found that when higher concentrations (generally above about 150 ppm) of one or more very strong oxidizers such as persulfate, permanganate, manganate, ozone hypochlorite, chlorate, nitric acid, iodine, bromine, chlorine, fluorine, or combinations of any two or more thereof that exist, or are formed, in a recirculation tank (or absorber recirculation tank—ART) slurry and/or solution, coupled with at a minimum a thermodynamically favorable pH and oxidation-reduction potential (ORP) in the wet scrubber, soluble manganese (Mn2+) forms MnxOy precipitate and impacts upon the nature, the amount and/or the conditions of mercury re-emission and selenium emission from a WFGD system. FIG. 2A is a Pourbaix diagram for manganese. At any point on the diagram it will give the thermodynamically most stable (and theoretically most abundant) form of that element at a given potential and pH condition. Of particular interest is the region for MnO2. The diagram shows that formation of MnO2 is favored as E(V) (i.e., ORP) increases in the pH range 5 to 6, the typical operating pH range for a wet scrubber. Note that approximately 200 mV must be subtracted from the y-axis to compare E(V) to measured ORP readings where a saturated Ag/AgCl reference electrode is used. Also of interest are the Pourbaix diagrams for mercury and selenium (see FIGS. 2B and 2C, respectively) as these elements and their various compounds and/or ionic species also need to be controlled in order to address various mercury reemission and selenium emission issues.
Also, it has been found that a portion of the precipitated MnxOy tends to collect on the walls of a wet scrubber below the liquid line in the lower half of the recirculation tank. When and where MnxOy collects on the wet scrubber walls made from Alloy 2205 (UNS S32205, a duplex stainless steel alloy), corrosion pitting has been observed to occur beneath the deposit. While not wishing to be bound to any one theory, a possible explanation for the corrosion mechanism is the MnxOy creates a galvanic cell with the wall alloy causing corrosion. In separate bench-scale corrosion experiments the presence of manganese dioxide (MnO2) has been shown to enhance corrosion but the creation of a galvanic cell not actually been proven.
Furthermore, in some instances it is also desirable to control the formation of various acidic ions that form in the presence of persulfate ions as they will react in the presence of calcium cations to form calcium sulfate and the corresponding halogen gas. This halogen gas will then further react in the slurry, or solution, of the ART to form, respectively, hypochlorite ions, hypobromite ions, and/or hypoiodite ions as illustrated by the exemplary equations below.S2O82−+2Cl−+2Ca2+→2CaSO4+Cl2 S2O82−+2Br−+2Ca2+→2CaSO4+Br2 S2O82−+2I−+2Ca2+→2CaSO4+I2 Cl2+H2O→2H++Cl−+ClO−Br2+H2O→2H++Br−+BrO−I2+H2O→2H++I−+IO−While not wishing to be bound to any one theory, the formation of hypochlorite ions, hypobromite ions, and/or hypoiodite ions is believed to negatively impact the pH and the ORP in the slurry, or solution, of an ART.
Given the above, a need exists in the art for a method and/or system by which to control manganese-based precipitates, as well as other corrosion related and/or unwanted precipitates, in the recirculation tank (or absorber recirculation tank—ART) of a wet flue gas desulfurization (WFGD) system. Additionally, a need exists in the art for a method and/or system that while permitting, or enabling, the achievement of one or more of the afore-mentioned goals, such a method and/or system will not adversely impact the amount, or type, of selenium and/or mercury in an environment typical of a WFGD. Furthermore, a need exists for a method and/or system that permits control of the oxidation-reduction potential in a bleed stream of an absorber slurry, filtrate, and/or solution from a wet flue gas desulfurization scrubber.